The present invention relates to systems and processes for coating solid surfaces with thin layers of a liquid, solution, or suspension and, more particularly, the fabrication of uniform layers of photoresist for lithography of semiconductor and other electronic materials.
Lithography of semiconductor materials such as silicon or gallium arsenide wafers requires the application of a very uniform, thin (typically less than 10.sup.-5 meter) layer of photoresist which is subsequently exposed to light through a mask followed by selective etching of either the exposed or unexposed material in a liquid bath or plasma. In order to obtain high resolution patterns, the photoresist must be less than 10 microns thick with minimal variation of thickness over the entire wafer.
A typical process employs spin-coating, in which a puddle or mist of liquid polymer solution is deposited onto a substrate, after which the substrate is spun at high speed (typically 1000-3000 rpm) so that shear forces lead to a uniform and thin film. An example of such a process is shown in Daraktchiev U.S. Pat. No. 4,996,080, which discloses a coating process in which a liquid photoresist composition is atomized, and the atomized spray droplets are permitted to settle by gravity onto a substrate surface, which is spun at between 500 and 6000 rpm. The spinning step may occur either simultaneously with the deposition of the atomized droplets, or subsequent to the deposition.
However, an inherent disadvantage with such coating processes is that it can waste as much as 95% or more of the photoresist solution, which cannot be recycled. Further, such spin-coating processes typically do not work well on substrates which are not circular, not flat, or are larger than a few tens of centimeters in diameter. Accordingly, there is a need for an apparatus and process for coating solid surfaces with thin layers of a liquid, solution or suspension which minimizes the amount of coating material wasted, and which can be employed satisfactorily on surfaces which are not circular, flat or smaller than a few tens of centimeters in diameter.